would a %20 rod and 20/20 focus stack?

say i 20% casting speed rod for healing prayers and a 20/20 focus for healing prayers, will they stack making it a 40% chance for healing prayers casting speed?

Where did you find the rod with 20% casting speed for healing prayers ? atm I'm after one aswell

Rather than stack to give you a 40% chance, it simply calculates the 20% chance twice, so, in a sense they stack, as in they both work, but they don't give you a 40% chance. That'd be really nice though =P

it's as Minwanabi says:

the chance of casting faster is 20/100 + 20/100 * 80/100 = 36/100, or 36% chance.

think of it like this: you have 2 items which give you probability of faster casting:
item A, and item B, each with 20% chance of working.

out of 100 casts, item A will have effect 20 times -- so we don't have to look at item B. of the remaining 80 casts where A did not have effect, item B had an effect 20% of the time -- 20% of 80 is 16... so item B had effect 16 times.

this gives a total of 36 out of 100 times that we got faster casting.

hope that makes sense.

possible, but is it also possible that item A and B act in the same time?

Yes, and thats why you get 36% instead of 40%. Both are doing the same thing, so one is in effect wasted .

Precisely. When you cast a spell it goes through two screens... each one being a 20% chance, if it passes both, it's normal, if either one stops it, you get the fast cast/recharge, but they won't stack to make one 40% chance.

So in a way to look at it, Its not 40% but 20% x2. So when ever you
cast a spell, it gives you a second chance of 20% to get it.
Well thats what i figure..... let me know if i'm wrong

Yeah that's pretty much it.

Spell Cast---20% check---if unsuccessful---20% check